Polyolefin microporous membranes are used as precision filter membranes, separators for batteries, separators for capacitors, materials for fuel cells, etc., and particularly, used as separators for lithium ion batteries. Recently, lithium ion batteries are used for miniature electronic instruments such as portable telephones and notebook type personal computers, and it is further attempted to apply them to electric cars, small motorbikes, etc.
Polyethylene microporous membranes are used as separators for lithium ion batteries because they are high in permeability, and besides they have a function to clog the communicating pores upon melting the polymer at 130-150° C. and to shut-down the current in order to ensure safety of the batteries. The term “shut-down” means a phenomenon that the pores of microporous membrane is clogged with molten resin to increase electrical resistance of the membrane to intercept the flow of lithium ions. When a microporous membrane is used as a separator of batteries, the shut-down temperature is desirably as low as possible from the viewpoint of ensuring the safety. Furthermore, as a function of separator, it is also necessary that the membrane maintains the shape of film after clogging of the pores to hold the insulation between electrodes. That is, if the temperature of battery further rises after shut-down of separators, fluidization of separator begins to cause decrease of electrical resistance, resulting in restoration of current. This temperature is called short-circuit temperature. Therefore, it is desired that the short-circuit temperature is higher.
With increase of function of small electronic instruments, increase of capacity of lithium ion batteries per unit volume is demanded. Thus, the higher safety at high temperatures is also required. For example, the lithium ion batteries are demanded to hold further safety at high temperatures considering extraneous heat generation of batteries, and simultaneously to maintain characteristics at high temperatures. In order to attain high capacity, it is a solution to thin the separators as much as possible and pack the electrodes as many as possible. When electrodes are packed, the inside of the battery is in the state of being densely packed, which causes the problem of low impregnation with electrolyte. Furthermore, when members of battery are wound, stronger mechanical load is applied to separators, and hence the separators are required to have a high mechanical strength even when they are thinned. Furthermore, it is also necessary to improve the output characteristics of the battery in the state of electrodes being densely packed, and hence a high permeability is also required for separators. Under the circumstances, there is demanded development of separators which have the above-mentioned functions, and furthermore high impregnation with electrolyte, mechanical strength, permeability, and storage characteristics at high temperatures when they are used in the batteries.
Patent Document 1 discloses a laminated sheet product comprising at least two layers in which a filler is contained in the second layer at a high content for the purpose of developing a high heat resisting temperature as a separator. However, according to the technology disclosed in this document, the separators can hardly be thinned, and it is difficult to attain increased capacity of battery per unit volume. Moreover, in the examples of Patent Document 1, large agglomerates are removed by extruding through two wire nets of 40 mesh. According to this method, since agglomerates passing through the nets are present, the portions where the agglomerates which have passed through the nets are present are low in mechanical strength to cause occurrence of pinholes. Furthermore, since agglomerates are removed, there sometimes occurs local ununiformity in composition. Therefore, the task is to improve uniformity of membrane.
Patent Document 2 discloses a separator in which a layer of an inorganic material-containing porous membrane comprising a polyolefin resin and an inorganic powder and a layer of an organic porous membrane comprising 100% by mass of a polyolefin resin are alternately laminated. In this document, impregnation with electrolyte, shut-down characteristics and heat resistance are attained. However, the inorganic material-containing membrane has a thickness of 10-20 μm, and the organic porous membrane is obtained by subjecting the inorganic material-containing porous membrane to extraction and removal with an alkali solution or the like. Therefore, further thinning of the separator is difficult, and attainment of increase in capacity of battery per unit volume is difficult. Moreover, only such an organic porous membrane as high in porosity can be produced, and hence the membrane strength tends to decrease. Further, in the examples of Patent Document 2, the separator is produced by superposing the inorganic material-containing porous membrane and the organic porous membrane, and hence the layers of the separator do not closely contact with each other, and the separators may slip off during fabrication of battery.
Patent Document 1: Japanese Patent No. 2108985
Patent Document 2: JP-A-2001-266828